Plasma arc machining method utilizing the high density heat of a physically, electrically converged plasma column has been widely practically used since it enables high-precision, high-efficiency cutting. In plasma arc machining method, cutting is usually performed by generating a plasma arc from a plasma torch and moving the plasma torch while stably maintaining the plasma arc.
The plasma torch has an electrode and a nozzle disposed so as to enclose the electrode, and a plasma gas is jetted to an object material through a path defined by the electrode and the nozzle. For cutting an object material with the plasma torch, a pilot arc is first generated between the electrode and the nozzle and is then grown into a main arc, whereby a plasma arc having high temperature and high-density energy is established between the electrode and the object material.
There are generally known two ways of cutting a steel plate, for instance, into various shapes with a plasma arc generated from the plasma torch. In one method, cutting is started from an end of a steel plate. Another method is such that a through hole is formed at a desired position of a steel plate and cutting is started from the through hole. The latter method is called “piercing start” and prevailing in the field of automatic cutting by use of an NC device because a workpiece of a desired shape can be cut out of a steel plate with this method.
In cases where a steel plate is cut by the piercing start, during piercing operation in which a through hole is made at a cutting start position by a plasma arc, the metal melted by the plasma arc is blown up in the form of spatter (molten metal droplets) and this blown spatter is likely to adhere to the nozzle. The spatter adhered to the nozzle could be a cause of melting-damage to the nozzle or occurrence of a double arc, which gives damage to the nozzle, resulting in a considerable decrease in cutting quality.
As an attempt to avoid such damage to the nozzle caused by spatter, there has been generally used a piecing technique as illustrated in FIG. 5(a) ({circle around (1)} to {circle around (4)}) to According to this technique, a plasma torch 51 is first moved to a level h1 that is the highest possible position for ignition of a steel plate W by a plasma arc ({circle around (1)}) and piercing is carried out at the level h1 ({circle around (2)} to {circle around (3)}). After the piecing is completed and therefore blowing-up of spatter is ceased, the plasma torch 51 is lowered to a level h2 suited for cutting operation and then, cutting is started ({circle around (4)}). A piercing method improved over the above conventional technique has been proposed in Japanese Patent Kokai Publication No. 2000-351076. The piercing method (hereinafter referred to as “raising piercing method”) disclosed in Publication No. 2000-351076 is carried out in the manner illustrated in FIG. 5(b) ({circle around (1)} to {circle around (4)}). According to this, the plasma torch 51 is first moved to a level H1 that is the upper limit position where ignition of a steel plate W by a plasma arc is possible and the steel plate W is ignited by the plasma arc at the level H1 ({circle around (1)} to {circle around (2)}). At the same time that piercing starts, the plasma torch 51 is raised a specified distance to a level H2 within the range that the plasma arc can be maintained in order to avoid damage to the nozzle caused by blowing-up of spatter, and then piercing is carried out at the level H2 ({circle around (3)}). After completion of the piercing operation, the plasma torch is lowered to a level H3 suited for cutting operation and subsequently, cutting is started ({circle around (4)}).
Where the level at which a plasma arc is first formed is defined as “initial level”, the level at which piercing is carried out is defined as “piercing level”, and the level at which cutting is carried out is defined as “cutting level”, the initial level h, is equal to the piercing level and higher than the cutting level h2 in the typical piercing method shown in FIG. 5(a). In the raising piercing method shown in FIG. 5(b), the initial level H1 is lower than the piercing level H2 and higher than the cutting level H3.
Incidentally, damage to the nozzle is caused by not only adhering spatter described above but also the pilot arc. Specifically, since the pilot arc has high-density heat energy like the main arc, the longer the pilot arc is generated, the more the nozzle gets damaged by melting. A technique for solving such a problem is disclosed in Japanese Patent Application No. 2002-021284 that has been previously filed by the present applicant. The technique of this prior invention is such that switching between a pilot current circuit for forming a pilot arc and a main current circuit for forming a main arc is transistorized and an adequate resistor is inserted in the pilot current circuit, thereby transferring the pilot arc into the main arc at higher speed so that melting-damage to the nozzle caused by the pilot arc is restrained.
The above-described typical piercing method and raising piercing method can mange to avoid the damage to the nozzle caused by spatter generated during piercing but have revealed the problem that the pilot arc causes the nozzle to deteriorate. More specifically, in these piercing methods, since the initial level is set to a relatively high value (about twice the cutting level), (A) a higher pilot current becomes necessary when the pilot arc is grown into the main arc; and (B) the electric resistance of the discharge path between the electrode and the steel plate is extremely higher than that of the discharge path between the electrode and the nozzle and therefore, an influx of current into the nozzle is likely to occur, retarding the transfer of the pilot arc into the main arc. For the reasons (A) and (B), the nozzle is excessively melting-damaged by the pilot arc. In addition, setting of the initial level high brings about an arc ignition defect. It is necessary to set the pilot current high in view of (A), but if the pilot current cannot be properly adjusted and therefore remains low, the pilot arc becomes weak so that the pilot arc does not grow into the main arc, resulting in a failure in arc ignition. Although it is conceivable to fixedly set the pilot current to the highest possible value that the system can output in order to save labor in adjusting the pilot current, melting-damage to the nozzle caused by the pilot arc is promoted all the more in this case.
In the above-described technique associated with the prior invention, a nozzle deterioration preventing effect can be expected to some extent owing to the high-speed transfer of the pilot arc to the main arc, but there is still room for improvement because the initial level is set to a relatively high value (about twice the cutting level).
The present invention has been made taking account of the foregoing problems and situations, and a primary object of the invention is therefore to provide a plasma arc machining method capable of avoiding the damage to the nozzle caused by spatter generated during piercing and reliably restraining the deterioration of the nozzle caused by the pilot arc, whereby the service life of the nozzle is significantly improved.